1. Field of the Invention
The present invention relates to an automatic density adjusting device in a copying machine which is so adapted as to allow copying at proper density by automatically detecting the brightness of a document to change exposure intensity or the like.
2. Description of the Prior Art
Conventionally, a copying machine with an automatic density adjusting function has been provided and has spread widely. The copying machine with an automatic document adjusting function has the advantage that time and labor required to manually set image density can be omitted to allow copying at proper density simply by a person which is not accustomed to operating the copying machine.
This automatic document adjusting function is one of illuminating and scanning (pre-scanning) a document once prior to copying operations to detect the brightness of the document, automatically calculating exposure intensity, the amount of charges or the amount of developing bias (hereinafter represented by "exposure intensity") which correspond to the detected brightness of the document on the basis of a predetermined relational expression and illuminating the document depending on the exposure intensity to obtain copies having proper density.
There has been also a method of reading the brightness of a document while illuminating the document without pre-scanning the document and calculating exposure intensity or the like most suitable for the brightness of the document at each time point.
In the above described copying machine with an automatic document adjusting function, the automatic document adjusting function as well as the conventional manual density setting function are made use of so that the automatic document adjusting function is performed when a user selects automatic document adjustment, while the manual density setting function is performed when the user selects manual setting (for example, a copying machine DC2520 manufactured by Mita Industrial Company, Ltd.).
Meanwhile, a copying machine making automatic document adjustment by omitting the manual density setting function has been recently considered. In such a copying machine, there is no manual density setting function, thereby to make it impossible to obtain copies having density depending on a user taste. Therefore, the copying machine is provided with a "density correcting function" of setting the image density to higher density or lower density depending on the user taste without setting to a one-to-one relationship between the brightness of the document and the most suitable exposure intensity or the like to a one-to-one relationship at the time of making the automatic document adjustment.
In such a density correcting function, it is generally considered that density correction curves in the shape of a straight line as shown in FIG. 2 are employed so as to make density corrections. In FIG. 2, the horizontal axis indicates density correction values, and the vertical axis indicates exposure intensity. When there is no density correction (the density correction value =0), exposure intensity to a newspaper which is representative of a dark document is expressed by I.sub.1, and exposure intensity to an NTC (New Test Chart) which is representative of a light document is expressed by I.sub.2. It goes without saying that a density correction curve which is intermediate between the curves is applied to a document of arbitrary brightness between the newspaper and the NTC.
If density corrections are made using the above described density correction curves, the difference between the exposure intensity to the newspaper and the exposure intensity to the NTC is always kept constant.
If the above described density correction curves are employed, the following problems arise.
Specifically, if the light document such as the NTC and the dark document such as the newspaper are subjected to density corrections, the exposure intensity to the light document and the exposure intensity to the dark document are changed while maintaining the same difference therebetween. Accordingly, the difference in density of a copy image hardly appears with respect to the dark document. That is, the document image is not too light even if the dark document is subjected to a "lighter" density correction, while the document image is not too dark even if the dark document is subjected to a "darker" density correction. This is caused by nonlinear characteristics of a photosensitive material or a developing agent. On the other hand, if the slope of the straight line is increased so that the difference in density of the copy image clearly appears with respect to the dark document, the light document is too light or too dark this time.
Furthermore, the copying machine is generally provided with a magnification setting function of manually setting magnification.
The magnification setting function is achieved by changing the position of a lens for directing a document image to a photoreceptor. However, the brightness of the image is inevitably changed as the magnification is changed. Accordingly, the change in density of this image must be corrected. Also in this case, therefore, the copying machine must be provided with a "density correcting function".
Conventionally, a density correction curve as shown in FIG. 4 has been generally employed. In FIG. 4, the horizontal axis indicates magnification (%), and the vertical axis indicates exposure correction values. Exposure intensity in a case where magnification is 100% is used as the basis, to increase the exposure intensity if the magnification is increased, while decreasing the exposure intensity if the magnification is decreased. The rate of the change in the exposure intensity is constant irrespective of the brightness of the document.
If the above described density correction curve is employed, the following problems arise.
Specifically, if the light document such as the NTC and the dark document such as the newspaper are subjected to density corrections at the same slope, the exposure intensity to the light document and the exposure intensity to the dark document are changed while maintaining the same difference therebetween. Therefore, with respect to the dark document, a copy image is liable to be dark if the magnification is increased, while being too light if the magnification is decreased. This is caused by nonlinear characteristics of a photosensitive material or a developing agent. On the other hand, if an attempt to cause the dark document to have proper density is made, an image of the light document is too light when the magnification is high, while being too dark when the magnification is low this time.
Consequently, an attempt to realize correct density corrections in the copying machine with an automatic document adjusting function or a magnification setting function makes the shape of a density correction curve important.